Make the Sweet Levain (Night Before): Mix all ingredients until a paste forms. Cover and let ferment at 74–78°F (23–26°C) for 10–12 hours, or until doubled and domed.
Make the Tangzhong: Whisk milk and flour together in a small saucepan. Cook over medium/low heat, stirring constantly, until it thickens into a paste (about 2–3 minutes). Scrape into a bowl and let cool.
Make the Dough: In a bowl, combine tangzhong, sweet levain, eggs, milk, vanilla, salt, and sugar. Add flour and mix until a shaggy dough forms. Knead for 10–15 minutes until smooth and elastic. Place in a greased bowl, cover, and bulk ferment at warm room temp for 5-6 hours.
Make the Cinnamon Filling: In a medium bowl, mix flour, brown sugar, cinnamon, and salt. Set aside.
Shape the Rolls: Roll dough into an 18x12-inch rectangle. Spread butter over dough, then spread cinnamon filling. Roll tightly and slice into 6 equal rolls. Place in a greased pan.
Final Proof: Cover and let rise at warm room temp for 2 hours, or until puffy.
Cream Bath & Bake: Preheat oven to 350°F (175°C). Pour heavy cream over rolls before baking. Bake for 35-40 minutes until golden and set.
Make Cream Cheese Glaze: Beat cream cheese and butter until smooth. Add vanilla and salt; beat to combine. Add powdered sugar and beat until creamy. Drizzle over warm rolls.
